The present invention relates to a device for measuring distance using a semiconductor laser in the visible wavelength range in accordance with the echo time method, the distance between the range finder and the sighted object being determined from the duration of the time elapsing between the transmission and reception of a laser modulation signal as a value for the distance.
Various range finders are known, which operate using lasers, for example, as distance measuring systems in motor vehicles or as level meters in silos. The systems usually operate using infrared pulse laser diodes of a very high-performance type, in which the pulse echo time to the object and back is measured.
German Published Patent Application No. 43 16 348 describes a portable laser range finder that operates using visible laser light. The distance is determined using the echo time method. In this context, the distance between the range finder and the sighted object is determined from the duration of the time elapsing between the transmission and reception of a laser pulse reflected by the object, as a value for the distance. The laser beam is bundled into a measuring beam bundle using a collimation objective; a circuit arrangement for modulating the measuring beam, a receiving objective for receiving and imaging the measuring beam bundle reflected by the distant object onto a receiving device, and an evaluation device for determining and displaying the distance measured to the object, are provided. Calibration is accomplished by a mechanical deflector shutter, which generates as a reference distance a light path of a known length. The coupling of the light received from the receiving lens into the photo diode takes place via a fiber-optic guide, whose entry surface, in accordance with one exemplary embodiment, is electromechanically adjusted to the distance-dependent focus. In the range finder, the emphasis lies in the optomechanical configuration, i.e., on the optically possible focusing in the receiving diode. Nothing is asserted concerning the type of evaluation and the evaluation method used.
In German Patent No. 4303 804, a laser range finder is described, in which the laser beam is modulated using two aliquant frequencies of the same order of magnitude, one after the other. In this context, an object is measured by the transmission light beams modulated using both modulation frequencies, one after the other. The two measuring values are compared in an evaluation unit to determine the distance of the object. The change in the modulation frequencies takes place as a function of the speed of the observed object.
Furthermore, a laser range finder is described in German Patent No. 44 11 218, the distance here also being determined according to the echo time principle. The emphasis in this document lies in the configuration of the electrical switchover between a reference and a receiving diode, in order to combine the two beams as early as possible and to avoid drift problems.
A device according to the present invention for measuring distance has an advantage in that the distance measurement is carried out in two operating modes. The first operating mode yields a larger uniqueness range and a smaller resolution. In the second operating mode, a high resolution is attained at a smaller uniqueness range. The combination of the two operating modes assures a large uniqueness range and high resolution. In this context, the expense for the modulation and for the evaluation is small, as a result of the skillful selection of parameters.
In principle, this is achieved through the present invention as a result of the fact that the circuit arrangement, for modulating the measuring beam, and the evaluation device can be switched between two different operating modes, the modulation taking place in the first operating mode, using a first frequency f1/n and, in the second operating mode, using a second frequency f1, which have a whole number ratio n to each other, and that the reflected signal in the first operating mode is multiplied using a third frequency f2/n and in the second operating mode using a fourth frequency f2, before they are sampled for evaluation purposes, in the first operating mode using a first sampling frequency fa1 and in the second operating mode using a second sampling frequency fa2, first sampling frequency fa1 being the difference between fourth frequency f2 and second frequency f1, and second sampling frequency fa2 being a whole-number fraction q of second frequency f1.
In an exemplary embodiment of the device according to the present invention, it is provided that the reflected signal mixed through multiplication in an analog mixer is filtered, in the first operating mode, in a first band pass using a first band-pass center frequency xcex94f/n, and in the second operating mode, in a second band pass, using a second band pass center frequency xcex94f, the two band pass center frequencies standing in a whole-number ratio n to each other.
In another exemplary embodiment according to the present invention, an analog/digital converter is provided, using which the sampling of the mixed reflected signal is carried out.
In a further simplification and easing of the evaluation, according to another embodiment according to the present invention, it is provided that in the second operating mode divider q is selected such that a subsampling is achieved using second sampling frequency fa2 of the mixed, reflected, and possibly band-pass filtered signal at second center frequency xcex94f, and specifically such that, as a result of aliasing in the analog/digital converter, second center frequency xcex94f, divided into the second sampling frequency, is represented by whole number n fa2/n, the following applying:
In an embodiment according to the present invention, a microprocessor is provided as a control computer for the evaluation.
In another embodiment according to the present invention, the phase measurement of the reflected, mixed, and preferably band-pass filtered, as well as sampled, signal is carried out using a discrete Fourier transformation in the control computer.
In yet another embodiment of the device for the measurement of distance according to the present invention, a digital mixer is provided, for the measurement of distance, a digital mixer is provided, in which, as a reference signal for the control computer, a signal is generated that marks the temporal position of the specific first of n sampling values. The evaluation and sampling is particularly simple when for n the value 4 is adopted, because in that case for the Fourier transformation only addition and subtraction are required. This is also possible using a simple microprocessor as the control computer.
In another embodiment according to the present invention, it is provided that, as a receiving device, a photo diode is provided which can be switched over by the control computer such that, alternately, it receives either the beam from the measuring object or, via a calibration segment, the beam from the measuring beam bundle. It is advantageous if the switchover takes place electrically, and a second photo diode is provided.
In an embodiment according to the present invention, it is provided that the control computer undertakes the switchover into the calibration mode automatically in response to each measurement.
It is advantageous that the phase measurement in the calibration phase is also carried out using a discrete Fourier transformation in the control computer. In this context, in a simple and advantageous configuration of the control computer, the phase value measured in the calibration phase is subtracted from the phase value determined in the measurement.
In a particularly advantageous and expedient design of the device according to the present invention, it is provided that, as the receiving objective, a Fresnel lens, in particular, made of plastic, is provided, and that in this Fresnel lens a thin opaque (that is, light-tight) tube is provided, outside the optical axis of the receiving objective, through which the collimated measuring beam bundle emerges. As a result of this configuration, it is assured that no optical cross talk between transmitter and receiver arises, and as a result of the parallax arising in this manner in the local environment, a too powerful increase of the receiving signal reflected by the measuring object is prevented.